判断单链表是否存在环，并找出环的入口

bool IsExitLoop(list *head)
{
    list *slow = head, *fast = head;

    while (fast && fast->next)
    {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast) break;
    }

    return !(slow == NULL || fast == NULL);
}

//找出环的入口
list *FindLoopPort(list *head)
{
    list *slow = head, *fast = head;

    while (fast && fast->next)
    {
        slow = slow->next;
        fast = fast->next->next;

        if (slow == fast) break;
    }

    if (slow == NULL || fast == NULL)
        return;

    slow = head;

    while (slow != fast)
    {
        slow = slow->next;
        fast = fast->next;
    }

    return slow;
}